1. Field of the Invention
The present invention concerns a tunnel effect sensor intended to be used in atomic force microscopes, for example.
2. Description of the Prior Art
A sensor of this type is described in patent EP 0 262 253 and is shown in FIG. 9 of the document in particular.
In this case, the sample to be examined is placed on a base mobile along three perpendicular axes X, Y and Z. The measurement head includes a tactile point that follows the surface to be examined and which is carried by a cantilever arm. The free end of the latter can be moved relative to a fixed arm parallel to the cantilever arm by an electrostatic force generated by respective capacitor plates on the two arms. Protuberances are provided at the free ends of the latter between which a tunnel current is established that can be varied by varying the distance between the two protuberances by means of a voltage established between the capacitor plates.
To examine the topography of the surface of a sample, the tactile point is brought close to the sample by moving the sample-holder base along the Z axis, for example. The distance between the tactile point and the surface to be examined is then adjusted precisely to cause the tactile point to interact with the surface to be examined, the force of interaction being reflected in changes in the tunnel current flowing between the two protuberances. The surface to be examined is then explored by moving the sample along the X and Y axes.
If the tactile point encounters a hollow or an asperity, the cantilever arm is deflected, which modifies the distance between the two protuberances and consequently the tunnel current between them. The latter being compared continuously with a set point value, an error signal is generated.
The latter is then used to move the sample-holder along the Z axis to compensate the error. The feedback signal needed for this is also used as a parameter representing the corresponding local "height" of the surface to be examined.
By scanning this surface in this way, the successively registered feedback signals can be used to trace curves of the level of this surface.
It can therefore be seen that the distance between the two protuberances of the beams (typically in the order of one nanometer) must be constantly corrected by commanding movement of the sample-holder along the Z axis. This is an extremely severe constraint on the precision of the device for moving the sample-holder, which must also move the latter along the other two axes (X or Y) with great precision.
This constraint is all the more severe in that the distance between the point at which the tunnel effect is produced and its electrode can vary by only a few nanometers for the tunnel effect to be measurable.
Another drawback of the prior art device is the fact that the protuberances may collide in the event of an unfortunate exaggerated movement of the sample-holder, which could seriously damage the sensor.
Finally, the set of beams is placed in front of the sample-holder and therefore blocks the field of view below the tactile point so that convenient visual observation by means of the usual instruments of the region in which the sample and the sample-holder are located is made difficult.
An aim of the invention is to provide a tunnel effect sensor that does not have the disadvantages just mentioned.